Jan 26, 2007 16:09
Nikkei Electronics Asia
Technology start-up Shimei
Semiconductor Co Ltd of Japan has developed a GaN light emitting diode
(LED) with high optical output - but on a Si substrate. The optical
output of the blue LED is 10mW max at 20mA, which is on a par with the
most-commonly used LEDs on sapphire substrates.In general, LEDs can be
manufactured on Si substrates for lower cost, for reasons including (1)
large-diameter Si wafers are suited to volume production, (2) they are
cheaper than sapphire substrates, and (3) they are more flexible and
easier to work than sapphire. In spite of these advantages, sapphire
substrates are widely used because Si substrates are more susceptible
to crystal lattice defects, which can degrade optical output. To resolve this problem, Shimei
Semiconductor replaced some of the GaN with AlInGaN, preventing lattice
defects from occurring. The developed LED has an emission wavelength of
450nm, and uses AlInGaN in the emission layer. The chip measures 0.3mm
square. The firm also makes green LEDs on Si substrates. The blue LEDs are scheduled to
sample-ship in April 2007, and the firm is ramping up for 3-million
units/month production. In addition to bare chips it is also
considering providing them on wafers. They currently use 2-inch Si
wafers.AlInGaN
Buffer Layers
When a GaN film
is grown on a Si substrate, the GaN film is susceptible to cracks,
displacement and other defects because of the large differences in
characteristics, including coefficient of thermal expansion and lattice
constant. Shimei Semiconductor placed an AlInGaN buffer layer between
the Si substrate and the n-type GaN layer to ameliorate this (Fig a).
There are several dozen layers of AlInGaN with varying compositions. The higher output involved not
only reducing lattice defects, but also minimizing the absorption of
output from the emission layer by the Si substrate. The multi-layer
AlInGaN buffer contains a distributed Bragg reflector (DBR), which
boosts optical output by about 1.5 times. The DBR consists of alternating
layers of two materials with different refractive indices, reflecting
specific wavelengths. The composition of the AlInGaN was varied to
create layers with different refractive indices. It would also have
been possible to use a metal mirror DBR, but the firm chose this
approach because it could all be formed on a film growth system. Another measure to minimize
lattice defects was optimization of growth parameters such as
temperature and pressure in the metal organic chemical vapor deposition
(MOCVD) process used to fabricate the LEDs. The LED chip film
(excluding the substrate) is 4 to 5um in thickness, relatively thick,
for the same reason. The n-type electrode was mounted at the base of
the LED chip, making it a vertical structure with electrodes top and
bottom. LEDs mounted on sapphire substrates grind down the top LED layer for the n-type
electrode because sapphire is an insulator. With Si, however, no
grinding process is needed, contributing to lower cost. The company also hopes to create
a white LED of AlInGaN by about 2010. Engineers hope to create red (R),
green (G) and blue (B) emission layers of AlInGaN on a single chip, as
well as using the material for the n- and p-type layers (Fig b). By
using AlInGaN for the n- and p-type layers, the firm believes emission
efficiency will be higher than that of GaN.by Tadashi
Nezu
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When a GaN film
is grown on a Si substrate, the GaN film is susceptible to cracks,
displacement and other defects because of the large differences in
characteristics, including coefficient of thermal expansion and lattice
constant. Shimei Semiconductor placed an AlInGaN buffer layer between
the Si substrate and the n-type GaN layer to ameliorate this (Fig a).
There are several dozen layers of AlInGaN with varying compositions. 
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